Add ability to negotiate the MTU on BTLE Android

There are cases where peripherals use a larger MTU than the default MTU
on Android. This forces the MTU negotiation to always been done once
the service discovery has been done.

This patch requires Android API v21 (or Android v5+). If the
local Android version is below 5 this feature becomes a noop. The related
bug cannot be addressed on Android version below 5.0.

Task-number: QTBUG-61755
Change-Id: I6521b5dad05da5e3e533ef2af56ee649b1b79730
Reviewed-by: Timur Pocheptsov <timur.pocheptsov@qt.io>